Prepare a bunch of genai packages for towncrier (#4763)

* Convert util/opentelemetry-util-genai to towncrier

* opentelemetry-instrumentation-openai-v2: convert changelog to towncrier

* Cleanup list of independently released packages

Drop genai packages that has never been released already and also
opentelemetry-exporter-credential-provider-gcp that is released with the
others.

* Apply suggestion from @xrmx

* Apply suggestion from @xrmx

* Fix pr number

* Bump openai-v2 util-genai dependency to >0.4b0,<0.6b0